We are sitting in a pub around the corner from the west London flat Garai – the star of BBC Two’s Fifties television newsroom drama The Hour – shares with her actor-director boyfriend Sam Hoare, and I have made the mistake of asking whether she takes much pleasure in wearing the fabulous, figure-hugging frocks in which her character, Bel Rowley, dresses.

For a second, she looks appalled. “I would be standing on set dressed in those provocative clothes,” she says, “and I just couldn’t stop thinking how difficult it was to reconcile Bel’s desire to be taken very seriously by the men around her with the fact that she is wearing something that is so, like… badda-bing! Badda-bing!”

Garai plays a trailblazing producer in the series, which when it first arrived on our screens last year was somewhat misleadingly touted as a British equivalent to Mad Men, AMC’s majestic drama set in a Madison Avenue advertising firm in the Fifties and Sixties. If you tune in for the start of the second series of The Hour on Wednesday expecting the stately pace and epic scope of that American masterpiece, you may well come away disappointed. “The real difference between The Hour and the big American TV shows is that some of them run across 22-episode seasons and you really cannot achieve the same thing in the six hours we have for [each season of] The Hour,” says Garai. “We simply can’t fund shows like that in this country; it’s never going to happen.”

Yet though The Hour, created by Abi Morgan (who also wrote the recent Margaret Thatcher biopic The Iron Lady), may be a comparatively modest, light-footed affair, it has an appeal of its own that is hard to resist. It fairly rattles along, sketching out the romantic tensions between its characters, while at the same time unspooling a thriller-like plot, and evoking a heady nostalgia for a time when the problems of the nation – racial tensions, underworld crime, political corruption – could be exposed and healed by a bit of good honest journalism. No mean feat for six hours of television.

If the series bears any comparison with Mad Men it is in both its impeccable ensemble cast – Rowley’s colleagues in the newsroom are played with great charm by Dominic West, Ben Whishaw and Anna Chancellor among others – and an approach to the recreation of the fashions and design of the Fifties so painstaking that it borders on the fetishistic.

“Everything on set was real,” says Garai. “The studio cameras were genuine, functioning cameras from the Fifties; the lamps, the stationery.” At one point, while filming a scene in which her character is handed a telex, she glanced down at the piece of paper that had been given to her expecting to see a discarded page of script and instead was confronted with a detailed message carefully produced on an original Fifties telex machine.

Garai says the production team’s relentless pursuit of authenticity felt like proof that someone was taking the job as seriously as she does; until, that is, she was shown her character’s wardrobe. “In every archive image that I saw of women in that environment [on the staff of the BBC], they wore suits and they wore tweed,” she says. “In terms of British fashions, the period was much more connected to the Forties than it was in America. Over here, professional women would dress like their mothers.”

There was a sense, she says, that to succeed as a woman in a male-dominated profession in Fifties Britain “you had to convey an idea of matriarchal status and certainly not be a sex object – you would move yourself as far away as possible from that. So a more restrained wardrobe would definitely have been my choice, but I lost that battle,” she smiles wryly. “Although I did get to wear more suits in series two.”

Bel Rowley is that rare thing in screen fiction – a successful woman who is also openly vulnerable, emotional and not in the least bit masculine. Garai plays her with a sympathetic quaver of uncertainty in her voice, a far cry from the steeliness of, say, Meryl Streep in The Devil Wears Prada or Helen Mirren in Prime Suspect.

“Female ambition is such a complicated thing to play because it is an aggressive quality,” says Garai, “and people respond very badly to women exhibiting any kind of aggression. Actually when I play Bel at her most assertive, every single director we have had on The Hour has said to me, 'That was really great but can you do it again and make her a bit less aggressive?’ Yet I know if it was a male character that kind of ambition would be perceived as strength.”

Garai, who turned 30 in the summer, characterises her approach to her own work as “ambitious but lazy. As an actor you really don’t have much control over your career so if you have very specific ideas of what you want from it – like 'I will win an Oscar by the time I’m 30’ – well… that way madness lies.”

Besides, she says, “even if the end goal is getting to work with any director you want in the world, if what you have to do to achieve that is to go to the gym for two hours every day, I still sort of can’t be bothered – which doesn’t mean I don’t want those things, I am just not prepared to go through what you have to go through to get them… which probably reflects very badly on me,” she smiles apologetically. “But that’s just the kind of person I am.”

Garai was born in Hong Kong and lived there until she was six, when her father, a bank manager, was transferred back to Britain to run a high street branch in Chippenham outside Bath. She attended an independent girls school in Wiltshire, where a classmate’s aunt, a casting director, spotted her in a school play and offered her her first professional, non-speaking, role. “Then I got an agent, and suddenly I was working as an actor,” she says, “which was really weird and hadn’t been the plan at all.” She went on to read English at Queen Mary, University of London, but dropped out after a year, in 2000, to take a part in the TV film Last of the Blonde Bombshells.

For a while, at the start of the new millennium, it looked as though she might be on the verge of a major film career. She followed her role as Kate in Douglas McGrath’s star-studded adaptation of Nicholas Nickleby, in 2002, with an acclaimed lead performance in Tim Fywell’s big-screen version of Dodie Smith’s I Capture the Castle and, on the strength of the latter role, she landed a bona fide Hollywood lead part, opposite Mexican hearththrob Diego Luna in 2004’s Dirty Dancing: Havana Nights, a belated sequel to the Patrick Swayze hit of the Eighties.

She was still only 19 when she embarked, unaccompanied, on the nine-month shoot. It was not a happy experience. “I was definitely too young and I was very out of my depth,” she says now. “I didn’t really know how to control what I did or didn’t want to do. I was miserable.

“If you are a 19-year-old woman, there are very specific things that directors and the people in positions of power in the industry – who tend to be older men – are going to want you to be and do. They are not going to want some chatty, difficult, slightly spoilt girl. They want you to be… demure, I suppose, and that didn’t come naturally to me. As I have got older, people have been a bit less perplexed if I have had an opinion I want to express.”

Was there a lot of pressure on her to look a certain way? “Yes, loads,” she says. “And I am not sure that has really changed as I have got older. If anything it gets worse, because then you are a woman in your forties and they are still, like, 'We would really rather you looked like you were 25,’” she laughs drily. “If you are an actress in LA, on your 40th birthday they should just hand you the keys to the lunatic asylum.”

Over the past decade, she has largely steered clear of Hollywood, working instead for an enviable roster of British and European directors including Joe Wright (Atonement); Lone Scherfig (One Day) and Stephen Poliakoff (Glorious 39). She has also made regular appearances on stage and, more recently, on television: in addition to The Hour, she played a Victorian prostitute in the BBC’s adaptation of Michel Faber’s The Crimson Petal and the White last year.

However, she says, “the last thing I did was a sci-fi film. I never thought I would say that. It’s called The Last Days on Mars, but they might have to change that as apparently every film that’s got 'Mars’ in the title has been a huge flop.”

Garai and Hoare are expecting their first child in the Spring, a fact that she doesn’t reveal to me (she is fiercely guarded about her personal life) but which Dominic West, her co-star in The Hour, will blurt out in a press conference a few days later. Perhaps it’s no fluke that the pregnancy coincides with a time when her thoughts are turning away from acting towards writing and directing.

Her directorial debut, a 20-minute short called Scrubber “about a young mother on a housing estate who has a small child but still wants to go out and have sex and stuff” was shown in competition at the Edinburgh Festival earlier in the year and gets a selected cinema release this week, packaged up with five shorts by other directors.

Garai funded the film herself. “The budget was £12,000,” she says, “I saved up for it. I did two series of The Hour, that is how I paid for it. But it still felt really extravagant. I have never spent that much money in one go except on a property. We shot it on 35mm and the film stock alone cost £6,000.”

Now she’s looking to drum up funding for her first feature film. Might this be the time to pursue a well-paid role in a big-budget piece of Hollywood froth? “What,” she says, “play a part I don’t want to play in something I don’t believe in?” Her face crumples back into that appalled expression. “I’d rather quit acting and work in a shop”.

The Hour starts on BBC Two on Wednesday 14 November at 9pm. Scrubber is now showing as part of The Joy of Six in selected cinemas nationwide. Details: www.nbcq.co.uk